The present invention relates generally to an electronic ignition device for use in internal combustion engines, and more particularly to the improvement of a digital ignition timing control device of the type described in U.S. Pat. No. 3,908,616, issued to Takao Sasayama on Sept. 30, 1975, and assigned to the assignee of the present application.
In the prior device, there is provided an ignition timing control device in which a first pulse train of a first frequency is counted by a first counter during each period between signals generated corresponding to predetermined angular positions of a crankshaft of the engine and the value so counted during each one of said periods is transferred to a second counter in which the transferred value is decreased by pulses of a second pulse train of a second frequency. The second counter produces an ignition timing signal upon reduction of said transferred value down to a predetermined value. The pulse frequency of the first pulse train is changed in accordance with the desired operating condition of the engine. Changing of the frequency, in the embodiment of the above-referred U.S. Patent, was achieved by utilizing a divide-by-N counter, that is to say, a well-known N stage counter, and desired pulse trains were derived from corresponding stages thereof. The frequency of the thus obtained pulse trains is, therefore, inverse to the k-th power of two (k being a positive integer less than N), i.e., 1, 1/2, 1/4, . . . 1/k. Only from such pulse trains could the above-mentioned first pulse train be selected, so that the frequency which could be selected was very limited and a fine selection of the frequency was impossible.
On the other hand, a finer and more accurate control of the ignition timing is being required in connection with various problems, e.g., saving fuel, environmental pollution, etc.